Graphene nanoribbons and carbon nanotubes fabricated from SiC fins or nanowire templates

ABSTRACT

Semiconductor structures including parallel graphene nanoribbons or carbon nanotubes oriented along crystallographic directions are provided from a template of silicon carbide (SiC) fins or nanowires. The SiC fins or nanowires are first provided and then graphene nanoribbons or carbon nanotubes are formed on the exposed surfaces of the fin or the nanowires by annealing. In embodiments in which closed carbon nanotubes are formed, the nanowires are suspended prior to annealing. The location, orientation and chirality of the graphene nanoribbons and the carbon nanotubes that are provided are determined by the corresponding silicon carbide fins and nanowires from which they are formed.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/088,766, filed Apr. 18, 2011 the entire content and disclosure ofwhich is incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

The present disclosure was made with Government support under ContractNo.: FA8650-08-C-7838 awarded by the Defense Advanced Research ProjectsAgency (DARPA). The Government thus has certain rights to thisdisclosure.

BACKGROUND

The present disclosure relates to semiconductor structures and methodsof fabricating the same. More particularly, the present disclosurerelates to semiconductor structures including parallel graphenenanoribbons or carbon nanotubes, which can be used as device channels,oriented along crystallographic directions. The present disclosure alsorelates to methods of making such semiconductor structures in which thegraphene nanoribbons or carbon nanotubes are fabricated from a templateof silicon carbide (SiC) fins or nanowires.

In the semiconductor industry there is a continuing trend towardfabricating integrated circuits (ICs) with higher densities. To achievehigher densities, there has been, and continues to be, efforts towarddown scaling the dimensions of the devices on semiconductor wafersgenerally produced from bulk silicon or silicon-on-insulator (SOI).These trends are pushing the current technology to its limits.

Very Large Scale Integrated (VLSI) circuits are typically realized withMetal Oxide Semiconductor Field Effect Transistors (MOSFETs). As thelength of the MOSFET gate is reduced, there is a need to thin the SOIbody (channel) so the device maintains good short channelcharacteristics. Adding a second gate opposite the first gate, so thechannel is controlled from both opposite faces of the SOI body allowsadditional scaling of the gate length. The best short channel control isachieved when a gate-all-around the channel is used.

In view of the above, the semiconductor industry is pursuing graphene toachieve some of the aforementioned goals. Graphene, which is essentiallya flat sheet of carbon atoms, is a promising material for radiofrequency (RF) transistors and other electronic transistors. Typical RFtransistors are made from silicon or more expensive semiconductors suchas, for example, indium phosphide (InP). The measured mobility ofelectrons in graphene was found to be much higher than for InP or forsilicon.

With all its excellent electronic properties, graphene is missing abandgap, making it unsuitable for fabrication of digital devices.Transistors fabricated using graphene in the channel would haveI_(on)/I_(off) ratios of the order of 10 or less, with many more ordersof magnitude (I_(on)/I_(off) of approximately 10⁶) still required forproper function of such devices. It has been shown that bandgaps can becreated in graphene if fabricated in the form of nanoribbons or a closedcarbon nanotube (CNT). The size of the bandgap increases with decreasingwidth of the nanoribbon and for potential practical application thewidth of the graphene nanoribbons (GNR) has to be less than 10 nm,preferably less than 5 nm.

Fabrication of GNR has been demonstrated before on exfoliated graphenenanoflakes. The prior art for fabrication of GNR is based on patterningand etching, usually by RIE, of the graphene layer. Such techniques formnanoribbons with non-uniform and potentially damaged edges, forming lineedge roughness, LER, which deteriorates the electrical quality of theGNR.

CNT field effect transistors are known to have excellent characteristicshowever accurate placement of the CNTs required for making a very largeintegrated circuit is very challenging. While some progress has beenmade by oriented growth of CNTs, the achievable CNT to CNT pitch is ofthe order of a micron. As a benchmark, present day devices are made witha pitch of 50 nm (0.05 microns).

SUMMARY

The present disclosure addresses the FET scaling requirements by usinggraphene as the channel material. The use of a graphene sheet allows tofabricate a channel that is thinner than can be made today with SOI.Additionally, the devices disclosed in the present disclosure have adouble gate to further push scaling. Use of CNT channels, which can bethought as rolled up graphene, allows the fabrication of gate-all-arounddevices.

The present disclosure describes the fabrication of semiconductorstructures including parallel graphene nanoribbons or carbon nanotubesoriented along crystallographic directions. The achievable integrationdensity is equivalent to that obtained in state-of-the-art silicontechnology since the graphene nanoribbons or carbon nanotubes arefabricated from a template of silicon carbide (SiC) fins or nanowires.

In the present disclosure, SiC fins or nanowires are first provided andthen graphene nanoribbons or carbon nanotubes are formed on exposedsurfaces of the fins or the nanowires by annealing. In embodiments inwhich closed carbon nanotubes are formed, the nanowires are suspendedprior to annealing. The location, orientation and chirality of thegraphene nanoribbons and the carbon nanotubes that are provided in thepresent disclosure are determined by the corresponding silicon carbidefins and nanowires from which they are formed.

In one embodiment of the present application, a semiconductor structure(i.e., dual-channel finFET) is provided that includes at least onesilicon carbide fin located on a surface of a substrate. The disclosedstructure also includes a graphene nanoribbon located on each baresidewall of the at least one silicon carbide fin. The disclosedstructure further includes a gate structure oriented perpendicular tothe at least one silicon carbide fin. The gate structure also overlaps aportion of each graphene nanoribbon and is located atop a portion of theat least one silicon carbide fin. In the disclosed structure, theportion of the each graphene nanoribbon overlapped by the gate structuredefines a channel region of the semiconductor structure.

In another embodiment of the present application, a semiconductorstructure is provided that includes at least one silicon fin located ona surface of a substrate. The disclosed structure also includes asilicon carbide fin located on each bare sidewall of the at least onesilicon fin, and a graphene nanoribbon located on a sidewall of eachsilicon carbide fin. The disclosed structure further includes a gatestructure oriented perpendicular to each silicon carbide fin and the atleast one silicon fin. The gate structure also overlaps a portion ofeach graphene nanoribbon and is located atop a portion of each of thesilicon carbide fins and the at least one silicon fin. The portion ofthe each graphene nanoribbon overlapped by the gate structure defines achannel region of the semiconductor structure.

In a further embodiment of the present application, a semiconductorstructure is provided that includes at least one pair of spaced apartgraphene nanoribbons located on a surface of a substrate. This structurealso includes a first gate structure located on one sidewall of eachspaced apart graphene nanoribbon, wherein the sidewalls of each graphenenanoribbon containing the first gate structure are not facing eachother. The structure further includes a planarizing dielectric materiallocated adjacent the first gate structure, and at least a gate conductorof a second gate structure located between the at least one pair ofspaced apart graphene nanoribbons. In some embodiments, an upper portionof the gate conductor of the second gate structure can contact an uppersurface of the first gate structure.

In an even further embodiment of the present application, asemiconductor structure is provided that includes at least one suspendedcarbon nanotube located atop a surface of a substrate, and a gatestructure oriented perpendicular to the at least one suspended carbonnanotube. The gate structure surrounds a portion of the at least onesuspended carbon nanotube, and portions of the at least one carbonnanotube surrounded by the gate structure define a channel region of thesemiconductor structure.

The present disclosure also provides a method of forming a semiconductorstructure. The method includes providing at least one silicon carbidefin having at least bare sidewalls on a surface of a substrate. Agraphene nanoribbon is formed on each bare sidewall of the siliconcarbide fin by annealing at a temperature from 1200° C. up to, but notbeyond the melting point of the substrate in an ambient such as, but notlimited to diluted silane. At least a gate structure is formed adjacentthe graphene nanoribbon.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawing description, when the term “cross sectional” isused the corresponding drawings will show objects (or materials) thatare present in a cross-section plane. When the term “side-view” is usedthe corresponding drawings will show objects that are directly visibleat a right angle and may reside behind the cross-section plane.

FIG. 1 is a pictorial representation (through a cross sectional view)depicting a silicon carbide-on-insulator substrate that can be employedin one embodiment of the present disclosure.

FIGS. 2A-2D are pictorial representation (through cross sectional views)depicting one possible method that can be used in forming the siliconcarbide-on-insulator substrate shown in FIG. 1.

FIG. 3 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 1 after forming a hard mask on an uppersurface of the silicon carbide layer of the silicon carbide-on-insulatorsubstrate.

FIG. 4A is a pictorial representation (through a top down view)depicting the structure of FIG. 3 after forming a plurality of siliconcarbide fins that include a patterned hard mask thereon in at least oneregion of the substrate.

FIG. 4B is a pictorial representation through a side-view after thestructure of FIG. 4A was cut at the plane marked by B1-B2.

FIG. 5A is a pictorial representation (through a top down view)depicting the structure of FIG. 4A after forming graphene nanoribbons onbare sidewalls of each of the silicon carbide fins.

FIG. 5B is a pictorial side-view representation through cut B1-B2 shownin the top down view of FIG. 5A.

FIG. 6A is a pictorial representation (through a top down view)depicting the structure of FIG. 5A after forming a gate structureincluding a gate dielectric and a gate conductor on a portion of eachsilicon carbide fin which includes graphene nanoribbons on the sidewallsthereof.

FIG. 6B is a pictorial side-view representation through cut B1-B2 shownin the top down view of FIG. 6A.

FIGS. 7A-7B are pictorial representations illustrating that the type ofgraphene that can be formed on the sidewalls of the silicon carbide finsprovided in FIGS. 6A-6B is dependent on the surface orientation of thesilicon carbide fin.

FIG. 8 is a pictorial representation (through a cross sectional view)illustrating a silicon-on-insulator substrate including, from bottom totop, a handle substrate, a buried insulator layer and a silicon layerthat can be employed in another embodiment of the present disclosure.

FIG. 9 is a pictorial representation (through a cross sectional view)illustrating the silicon-on-insulator substrate of FIG. 8 after forminga hard mask on an upper surface of the silicon layer of thesilicon-on-insulator substrate.

FIG. 10 is a three dimensional representation of the structure shown inFIG. 9 after forming at least one silicon fin on an upper surface of theburied insulator layer, each silicon fin having a patterned hard masklocated thereon.

FIG. 11 is a three dimensional representation of the structure shown inFIG. 10 after forming silicon carbide fins on the bare sidewalls of thesilicon fin.

FIG. 12 is a three dimensional representation of the structure shown inFIG. 11 after forming a graphene nanoribbon on bare sidewalls of eachsilicon carbide fin.

FIG. 13 is a three dimensional representation of the structure shown inFIG. 12 after forming a first gate structure including a first gatedielectric and a first gate conductor thereon.

FIG. 14 is a cross sectional view of the structure shown in FIG. 13taken along the A1-A2 plane.

FIG. 15 is a pictorial representation (through a cross sectional view)illustrating the structure shown in FIG. 14 after forming a planarizingdielectric layer and planarizing the structure stopping on an uppersurface of the patterned hard mask.

FIG. 16 is a pictorial representation (through a cross sectional view)illustrating the structure of FIG. 15 after selectively removing thepatterned hard mask and the silicon fin from the structure, andformation of a second gate conductor in the area previously occupied bythe patterned hard mask and the silicon fin.

FIG. 17 is a pictorial representation (through a cross sectional view)illustrating the structure of FIG. 15 after selectively removing thepatterned hard mask and the silicon fin from the structure, andformation of a second gate dielectric and a second gate conductor (i.e.,a second gate structure) in the area previously occupied by thepatterned hard mask and the silicon fin.

FIGS. 18A-18B are pictorial representations (through cross sectionalviews) illustrating the structure of FIG. 15 after selectively removingthe patterned hard mask, the silicon fin and the silicon carbide finsfrom the structure and formation of a second gate dielectric and asecond gate conductor (i.e., a second gate structure) in the areapreviously occupied by the patterned hard mask, the silicon fin and thesilicon carbide fins.

FIG. 19A is a pictorial representation (through a top down view)depicting the structure of FIG. 1 after forming a plurality of suspendedsilicon carbide nanowires located in at least one region of thestructure.

FIG. 19B is a cross sectional view of the structure shown in FIG. 19Athrough cut A1-A2.

FIG. 20A is a pictorial representation (through a top down view) of thestructure shown in FIG. 19A after forming a graphene coating on allexposed surfaces of the plurality of suspended silicon carbidenanowires; the nanowires coated with graphene may be referred to hereinas carbon nanotubes.

FIG. 20B is a side-view of the structure shown in FIG. 20A through cutA1-A2.

FIG. 21A is a pictorial representation (through a top down view) of thestructure shown in FIG. 20A after forming a gate structure including agate dielectric and a gate conductor over a portion of each carbonnanotube.

FIG. 21B is a side-view of the structure shown in FIG. 21A through cutA1-A2.

DETAILED DESCRIPTION

The present disclosure, which provides semiconductor structuresincluding parallel graphene nanoribbons or carbon nanotubes, which canbe used as device channels, oriented along crystallographic directions,and methods of fabricating such structures, will now be described ingreater detail by referring to the following discussion and drawingsthat accompany the present application. It is noted that the drawings ofthe present application are provided for illustrative purposes only and,as such, the drawings are not drawn to scale. It is also noted that inthe drawings like and corresponding elements are referred to using likereference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present disclosure. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present disclosure may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present disclosure.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

As mentioned above, the present disclosure provides semiconductorstructures including parallel graphene nanoribbons or carbon nanotubes,which can be used as device channels, oriented along crystallographicdirections as well as methods of fabricating such semiconductorsstructures. The methods of the present disclosure, which will bedescribed in further detail herein below, form the graphene nanoribbonsor carbon nanotubes from a template of silicon carbide fins ornanowires. The location, orientation and chirality of the graphenenanoribbons and the carbon nanotubes that are provided in the presentdisclosure are determined by the corresponding silicon carbide fins andnanowires from which they are formed. As such, the methods of thepresent disclosure can be used in existing semiconductor processingflows and provide a technology in which a dense population of graphenenanoribbons and carbon nanotubes can be selectively placed on asubstrate.

Reference is first made to FIGS. 1, 2A-2D, 3, 4A, 4B, 5A, 5B, 6A and 6Bwhich illustrate one embodiment of the present disclosure in which adual-channel finFET including graphene nanoribbons is provided.

Referring first to FIG. 1, there is illustrated a siliconcarbide-on-insulator substrate 10 that can be employed in one embodimentof the present disclosure. The silicon carbide-on-insulator substrate 10shown in FIG. 1 includes, from bottom to top, a handle substrate 12, aburied insulating layer 14 and a silicon carbide layer 16.

The handle substrate 12 of the silicon carbide-on-insulator substrate 10may include any semiconducting material or insulating material such as,for example, Si, SiC, GaN, AlN, Al₂O₃, Si₃N₄ or other like compoundsemiconductors or metal oxides. The materials used for handle substrate12 typically have a melting point higher than 1200° C. Multilayers ofthese semiconductor materials can also be used as the semiconductormaterial of the handle substrate 12. In one embodiment, the handlesubstrate 12 is comprised of silicon. In another embodiment, the handlesubstrate 12 is comprised of silicon carbide.

The handle substrate 12 and the silicon carbide layer 16 of the siliconcarbide-on-insulator substrate 10 may have the same or different crystalorientation. For example, the surface crystal orientation of the handlesubstrate 12 and the silicon carbide layer 16 may be {100}, {110}, or{111}. Other crystallographic orientations besides those specificallymentioned can also be used in the present disclosure. The handlesubstrate 12 of the silicon carbide-on-insulator substrate 10 may be asingle crystalline semiconductor material, a polycrystalline material,or an amorphous material. Typically, the silicon carbide layer 16 of thesilicon carbide-on-insulator substrate 10 is a single crystallinesemiconductor material.

In one embodiment of the present disclosure, the handle substrate 12and/or the silicon carbide layer 16 of the silicon carbide-on-insulatorsubstrate 10 may be undoped. In another embodiment of the presentdisclosure, the handle substrate 12 and/or the silicon carbide layer 16of the silicon carbide-on-insulator substrate 10 are doped. When thehandle substrate 12 and/or the silicon carbide layer 16 of the siliconcarbide-on-insulator substrate 10 are doped, the dopant may be a p-typeor an n-type dopant.

The buried insulating layer 14 of the silicon carbide-on-insulatorsubstrate 10 may be an oxide, nitride, oxynitride or any multilayeredcombination thereof. In one embodiment, the buried insulating layer 14of the silicon carbide-on-insulator substrate 10 is an oxide such as,for example, silicon oxide, aluminum oxide, and silicon nitride. Theburied insulating layer 14 may be continuous or it may be discontinuous.When a discontinuous buried insulating layer 14 is present, the buriedinsulating layer 14 exists as an isolated island that is surrounded bysemiconductor material.

The thickness of the silicon carbide layer 16 of the siliconcarbide-on-insulator substrate 10 is typically from 0.5 nm to 10 nm,with a thickness from 1 nm to 5 nm being more typical. If the thicknessof the silicon carbide layer 16 exceeds the above mentioned ranges, athinning step such as, for example, oxidation followed by an oxidestripping, planarization or etching can be used to reduce the thicknessof the silicon carbide layer 16 to a value within one of the rangesmentioned above.

The buried insulating layer 14 of the silicon carbide-on-insulatorsubstrate 10 typically has a thickness from 1 nm to 200 nm, with athickness from 100 nm to 150 nm being more typical. In embodiments inwhich the handle substrate 12 is an insulator (such as Al₂O₃) there isno need for insulating layer 14. In this case, the substrate 10 maycomprise just the silicon carbide layer 16 over the handle substrate 12.However, in some cases layer 14 is used even when the handle substrate12 is an insulator. For example when substrate 10 is fabricated bybonding, it is sometimes hard to bond silicon carbide directly tosubstrate 12 and an intermediate insulating layer can be used as the“glue” between the silicon carbide and the handle substrate. Thethickness of the handle substrate 12 of the silicon carbide-on-insulatorsubstrate 10 is inconsequential to the present disclosure.

In one embodiment, the silicon carbide-on-insulator substrate 10 may beformed utilizing a process in which carbon ions are implanted into aSIMOX (Separation by IMplanted OXygen) wafer. In another embodiment ofthe present disclosure, the silicon carbide-on-insulator substrate 10 isformed by first providing a handle substrate 12. Next, the buriedinsulating layer 14 is formed on the handle substrate 12 and thereafterthe silicon carbide layer 16 is formed on the buried insulating layer14. To obtain a single-crystal SiC layer 16, formation of layers 12 and14 can be done by epitaxy. In yet a further embodiment of the presentdisclosure, the silicon carbide-on-insulator substrate 10 is formed bylayer transfer. When a layer transfer process is employed, an optionalthinning step may follow the bonding of a wafer including a handlesubstrate to a wafer including a silicon carbide substrate. The optionalthinning step reduces the thickness of the silicon carbide substrate toa layer having a thickness that is more desirable and within the rangesprovided above.

Reference is now made to FIGS. 2A-2D which illustrate the basicprocessing steps of a layer transfer process that that can be used inone embodiment of the present disclosure in forming the siliconcarbide-on-insulator substrate 10 shown in FIG. 1. Referring first toFIG. 2A, there is illustrated an initial structure 20 that can be usedin forming the silicon carbide-on-insulator substrate 10 shown inFIG. 1. The initial structure 20 includes a silicon carbide substrate 22having a first insulating layer 24 located on an upper surface thereof.The first insulating layer 24 includes one of the insulating materialsmentioned above for buried insulating layer 14. In one embodiment, thefirst insulating layer 24 can be formed by a thermal technique includingoxidation and/or nitridation. Alternatively, the first insulating layer24 can be formed on an upper surface of the silicon carbide substrate 22by a deposition process including, for chemical vapor deposition, plasmaenhanced chemical vapor deposition, atomic layer deposition, andchemical solution deposition.

Referring now to FIG. 2B, there is illustrated the structure of FIG. 2Aafter forming a hydrogen implant region 26 within the silicon carbidesubstrate 22. The hydrogen implant region 26 is formed utilizing anyconventional hydrogen ion implantation process. The hydrogen implantregion 26 includes a sufficient concentration of hydrogen ions that uponsubjecting the same to a subsequent annealing blistering occurs withinthe implant region 26 which removes a portion of the silicon carbidesubstrate 22 from the structure.

Referring now to FIG. 2C, there is illustrated the structure of FIG. 2Bafter providing a handle substrate 12 having a second insulating layer28 located on an upper surface thereof, flipping the structure shown inFIG. 2B and bonding the two wafers together by bringing the same inintimate contact with each other; in the embodiment illustrated thefirst and second insulator layers 24, 28 are brought into intimatecontact with each other. Bonding is typically initiated by van der Waalsforces between the two flat surfaces 24 and 28. Applying pressure on thetwo wafers can also be used to initiate bonding. Annealing is used tostrengthen the bond between the two wafers. After annealing the bondbetween the two surfaces is a covalent bond. Typical annealingtemperatures are from 300° C. to 1200° C., while the annealing durationis from 0.5 hours to 24 hours. As mentioned above, annealing also leadsto separation of a part of the silicon carbide substrate 22 due tohydrogen blistering that occurs in the hydrogen implant region 26. Theremaining silicon carbide which is not removed from the original siliconcarbide substrate 22 is then polished to obtain a silicon carbide layer16 whose surface has root mean square (RMS) roughness from 0.1 nm to 0.3nm. The resultant structure after polishing is shown, for example, inFIG. 2D. During bonding, the first and second insulating layers 24, 28can merge and form the buried insulating layer 14 of the siliconcarbide-on-insulator substrate 10.

Notwithstanding which process is employed in forming the siliconcarbide-on-insulator substrate 10 shown in FIG. 1, a hard mask 30 isformed on an upper surface of the silicon carbide layer 16 of thesilicon carbide-on-insulator substrate 10 providing the structure suchas shown, for example, in FIG. 3. The hard mask 30 employed in thepresent disclosure includes an oxide, nitride, oxynitride or anymultilayered combination thereof. In one embodiment, the hard mask 30 isa semiconductor oxide such as, for example, silicon oxide. In anotherembodiment, the hard mask 30 is a semiconductor nitride such as, forexample, silicon nitride. In yet a further embodiment of the presentdisclosure, the hard mask 30 includes a multilayered stack of asemiconductor oxide and a semiconductor nitride, i.e., a siliconoxide-silicon nitride multilayered stack.

In one embodiment, a thermal technique such as, for example, oxidationand/or nitridation can be used in forming the hard mask 30 on the uppersurface of the silicon carbide layer 16. In another embodiment, adeposition process such as, for example, chemical vapor deposition,plasma enhanced chemical vapor deposition, atomic layer deposition andchemical solution deposition can be used in forming the hard mask 30.

The thickness of the hard mask 30 may vary depending on the type of hardmask material employed and the technique used in forming the same.Typically, the hard mask 30 has a thickness from 5 nm to 50 nm, with athickness from 10 nm to 20 nm being more typical.

Referring now to FIGS. 4A-4B, there is illustrated the structure shownin FIG. 3 after forming a plurality of silicon carbide fins 16′ on thesurface of the buried insulating layer 14 of the siliconcarbide-on-insulator substrate 10. As shown, each silicon carbide finincludes a patterned hard mask 30′ thereon. The term “fin” is usedthroughout the present disclosure to denote a portion of either siliconcarbide or silicon that was etched out of a silicon carbide layer or asilicon layer. The fin has a rectangular cross-section, with the finheight being defined by the thickness of silicon carbide layer 16 andthe fin width being defined by the width of the patterned hard mask 30′.

It is noted that although the drawings and following description referto a plurality of silicon carbide fins, the present application also canbe employed when a single silicon carbide fin is formed. It is alsonoted that in the top down views, the silicon carbide fins 16′ arelocated beneath the patterned hard mask 30′.

The plurality, i.e., array, of silicon carbide fins 16′ is located in atleast one region of the silicon carbide-on-insulator substrate 10. Eachsilicon carbide fin 16′ has a bottom surface that is direct contact withan upper surface of the buried insulating layer 14 of the siliconcarbide-on-insulator substrate 10, a top surface in direct contact witha bottom surface of the patterned hard mask 30′ and bare sidewalls. Asis illustrated, each silicon carbide fin 16′ has a first end portion E1that is in contact with a first unpatterned portion of the siliconcarbide layer 16, and a second end portion E2 that is in contact with asecond unpatterned portion of the silicon carbide layer 16. As alsoillustrated, the plurality of silicon carbide fins 16′ are arrangedparallel to each other and a uniform space is present between eachneighboring silicon carbide fin 16′. The array of silicon carbide fins16′ can thus be considered as a ladder arrangement in which each siliconcarbide fin represents a rung of the ladder.

The structure shown in FIGS. 4A-4B can be formed by lithography andetching. Specifically, the structure shown in FIGS. 4A-4B can be formedby first applying a photoresist material (not shown) to the uppersurface of the hard mask 30. The photoresist material, which can be apositive-tone material, a negative-tone material or a combination ofboth positive-tone and negative-tone materials, can be formed utilizingany conventional deposition process including, for example, spin-oncoating. Following the application of the photoresist material, thephotoresist material is subjected to a desired pattern of radiation (forexample, optical illumination through a mask, or electron beamlithography) and thereafter the resist material is developed utilizingany conventional resist developer.

With the patterned resist on the surface of the hard mask 30, theunprotected portions of the hard mask 30 and underlying portions of thesilicon carbide layer 16 not covered by the patterned resist are removedutilizing one or more etching processes. The one or more etchingprocesses that can be used in removing the unprotected portions of thehard mask 30 and underlying portions of the silicon carbide layer 16 notcovered by the patterned resist include dry etching, wet etching or anycombination thereof. When dry etching is employed, one of reactive ionetching (RIE), ion beam etching, and plasma etching can be used. Whenwet etching is employed, a chemical etchant that is selective in removeunprotected portions of at least the hard mask 30 can be used. In oneembodiment of the present application, RIE can be used to remove theunprotected portions of the hard mask 30 and the underlying portions ofthe silicon carbide layer 16.

In some embodiments, the patterned resist remains atop the structureduring the entire patterning process. In other embodiments of thepresent disclosure, the patterned resist is removed from the structureafter the pattern has been transferred into the hard mask 30.Notwithstanding when the patterned resist is removed, the patternedresist is removed utilizing a conventional resist removal processingsuch as ashing.

Referring now to FIGS. 5A-5B, there is illustrated the structure ofFIGS. 4A-4B after forming graphene nanoribbons 32 on the bare sidewallsof each of the silicon carbide fins 16′. The term “nanoribbon” is usedthroughout the present application to denote a rectangular graphenesheet with one dimension being a few nanometers wide. It is noted thatin the top down view the nanoribbons are located on the sidewalls of thefins and are thus not visible.

Although not illustrated in the drawings, the present applicationincludes an embodiment in which at least each patterned hard mask 30′ isremoved from atop the silicon carbide fins 16′ prior to forming thegraphene nanoribbons. When the patterned hard masks 30′ are removed, agraphene nanoribbon can be formed on the bare sidewalls as well as thenow bare upper surface of each silicon carbide fin. It is noted that inthis case and for some applications, one may want to choose the finorientation such that the exposed SiC fin sidewalls and the exposed topsurface have the same crystal orientation (for example, all having a(100) surface).

The term “graphene” as used throughout the present application denotes aone-atom-thick planar sheet of sp²-bonded carbon atoms that are denselypacked in a honeycomb crystal lattice. The graphene employed as graphenenanoribbons 32 has a two-dimensional (2D) hexagonal crystallographicbonding structure. The graphene that can be employed as graphenenanoribbon 32 can be comprised of single-layer graphene (nominally 0.34nm thick), few-layer graphene (2-10 graphene layers), multi-layergraphene (>10 graphene layers), a mixture of single-layer, few-layer,and multi-layer graphene, or any combination of graphene layers mixedwith amorphous and/or disordered carbon phases. The graphene employed asgraphene nanoribbons 32 can also include, if desired, substitutional,interstitial and/or intercalated dopant species as well.

Each graphene nanoribbon 32 that is formed on the bare sidewalls of eachsilicon carbide fin 16′ can be formed by first cleaning the baresidewalls of each silicon carbide fin 16 by performing a first anneal ina dilute silane-containing ambient. The first anneal that can be used toclean the bare sidewalls of each silicon carbide fin 16′ is typicallyperformed at a temperature from 800° C. to 900° C., with a first annealtemperature from 810° C. to 825° C. being more typical.

As mentioned above, the first anneal is performed in a dilutesilane-containing ambient. By “silane-containing ambient” it is meantany atmosphere that includes at least one compound of hydrogen andsilicon that has the general formula Si_(n)H_(2n+2) wherein n is anyinteger, particularly n is from 1 to 4. Examples of silanes that can beemployed within the silane-containing ambient include, but are notlimited to, silane and disilane.

The silane-containing ambient is typically diluted with an inert gasincluding for example, at least one of He, Ne, Ar, Kr and Xe. In oneembodiment, the content of silane within the dilute silane-containingambient is typically from 1% to 100% based on the total amount of thedilute silane-containing ambient. In another embodiment, the content ofsilane within the dilute silane-containing ambient is typically from 15%to 25% based on the total amount of the dilute silane-containingambient.

After performing the first anneal, a second anneal is performed thatgrows graphene nanoribbons 32 on the bare sidewalls of each siliconcarbide fin 16′. For each silicon carbide fin 16′, two graphenenanoribbons are formed on opposing sidewall surfaces of the fin.Portions of each graphene nanoribbon will serve as the channel for thedevice. The second anneal is typically performed at a temperature fromabout 1200° C. up to, but not exceeding the melting temperature of thehandle wafer 12, with a second anneal temperature from 1300° C. to 2000°C. being more typical. During the second anneal, silicon is release fromthe bare sidewalls of the silicon carbide fins 16′ forming graphenenanoribbons thereon. The width of each graphene nanoribbon 32 that isformed is defined by the height of each silicon carbide fin 16′.Typically, the width of each graphene nanoribbon 32 is within a rangefrom 0.5 nm to 10 nm.

Referring now to FIGS. 6A-6B, there is illustrated the structure ofFIGS. 5A-5B after forming a gate structure 35 including a gatedielectric (not shown) and a gate conductor 34 on a portion of eachsilicon carbide fin 16′ which includes graphene nanoribbons 32 on thesidewalls thereof. The gate dielectric, which is not shown, is locatedbeneath the gate conductor 34 and atop the buried insulating layer 14.Further, the gate dielectric completely surrounds each silicon carbidefin 16′ that includes a graphene nanoribbon 32 on its sidewalls.

In one embodiment of the present disclosure, the gate dielectric thatcan be used in this embodiment can include a metal oxide or asemiconductor oxide. Exemplary gate dielectrics that may be use include,but are not limited to, HfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃,Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y),TiO_(x)N_(y), SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), SiON,SiN_(x), a silicate thereof, and an alloy thereof. Multilayered stacksof these dielectric materials can also be employed as the gatedielectric layer. Each value of x is independently from 0.5 to 3 andeach value of y is independently from 0 to 2.

The thickness of the gate dielectric that can be employed may varydepending on the technique used to form the same. Typically, the gatedielectric that can be employed has a thickness from 1 nm to 20 nm, witha thickness from 2 nm to 10 nm being more typical.

The gate dielectric can be formed by methods well known in the art. Inone embodiment, the gate dielectric can be formed by a depositionprocess such as, for example, chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), physical vapor deposition(PVD), molecular beam deposition (MBD), pulsed laser deposition (PLD),liquid source misted chemical deposition (LSMCD), and atomic layerdeposition (ALD). If the gate dielectric is a stack of several layers,some of the layers can be deposited by chemical deposition or a spin-ontechnique.

After forming the gate dielectric, the gate conductor, i.e., gate line,34 can be formed. The gate conductor 34 includes any conductive materialincluding, but not limited to, polycrystalline silicon, polycrystallinesilicon germanium, an elemental metal (e.g., tungsten, titanium,tantalum, aluminum, nickel, ruthenium, palladium and platinum), an alloyof at least two metals, a metal nitride (e.g., tungsten nitride,aluminum nitride, and titanium nitride), a metal silicide (e.g.,tungsten silicide, nickel silicide, and titanium silicide) andmultilayered combinations thereof. In one embodiment, the conductivematerial that can be employed as the gate conductor 34 can be comprisedof an nFET metal gate. In another embodiment, the conductive materialthat can be employed as gate conductor 34 can be comprised of a pFETmetal gate. The nFET and pFET gate conductors are chosen based on thedesired FET threshold voltage (Vt). In a further embodiment, theconductive material that can be employed as gate conductor 34 can becomprised of polycrystalline silicon. The polysilicon conductivematerial can be used alone, or in conjunction with another conductivematerial such as, for example, a metal conductive material and/or ametal silicide material.

The conductive material that is employed as the gate conductor 34 can beformed utilizing a conventional deposition process including, forexample, chemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD), physical vapor deposition (PVD), sputtering, atomiclayer deposition (ALD) and other like deposition processes. WhenSi-containing materials are used as the conductive material, theSi-containing materials can be doped within an appropriate impurity byutilizing either an in-situ doping deposition process or by utilizingdeposition, followed by a step such as ion implantation or gas phasedoping in which the appropriate impurity is introduced into theSi-containing material. When a metal silicide is formed, a conventionalsilicidation process is employed. The as-deposited conductive materialtypically has a thickness from 1 nm to 100 nm, with a thickness from 3nm to 30 nm being even more typical. Following deposition of theconductive material, the conductive material is patterned by lithographyand etching into gate conductor, i.e., gate line, 34. During thepatterning of the conductive material, the gate dielectric may also bepatterned as well.

Specifically, FIGS. 6A-6B illustrate a dual-channel finFET that includesat least one silicon carbide fin 16′ located on a surface of asubstrate, i.e., the buried insulating layer 14 of the initial siliconcarbide-on-insulator substrate 10. The disclosed structure also includesa graphene nanoribbon 32 located on each bare sidewall of the at leastone silicon carbide fin 16′. The disclosed structure further includes agate structure 35 oriented perpendicular to the at least one siliconcarbide fin 16′. The gate structure 35 also overlaps a portion of eachgraphene nanoribbon 32 and is located atop a portion of the at least onesilicon carbide fin 16′. In the disclosed structure, the portion of theeach graphene nanoribbon 32 overlapped by the gate structure 35 definesa channel region of the semiconductor structure.

The structure shown in FIGS. 6A-6B also includes a source region 38A anda drain region 38B. The source region 38A is located at one portion ofeach graphene nanoribbon which is not overlapped by the gate structure35, while the drain region 38B is located at another portion of eachgraphene nanoribbon which is not overlapped by the gate structure 35.The source region 38A and drain region 38B are connected by the channelregion.

In one embodiment, the source region 38A and the drain region 38B can beformed by chemical doping (n-type or p-type) of portions of the graphenenanoribbon 32 that are not overlapped by the gate structure 35. Forexample, graphene can be doped to be p-type by exposure to nitric acid.In another embodiment, the source region 38A and the drain region 38Bare composed of a metal carbide which is formed by first forming a metallayer such as Ti, W, Ni, Ta, Co or alloys thereof, on a portion of eachgraphene nanoribbon in which the source/drain regions 38A, 38B are to beformed. The metal layer and the graphene nanoribbon are then reacted byannealing. For example, to form tungsten carbide (WC) at a temperatureof about 900° C. or greater is needed. Following the anneal, anyunreacted metal layer can be removed utilizing a selective etchingprocess. Chemical vapor deposition with a metal precursor can also beapplied to form carbides.

It should be noted that the type of graphene that can be formed on thesidewalls of the silicon carbide fins in the present disclosure isdependent on the surface orientation of the silicon carbide fin. This isshown in FIGS. 7A-7B. Specifically, FIG. 7A is a drawing which showssome of the possible crystal planes of a silicon carbide-on insulatorsubstrate with a notch in the (101) direction. As shown in FIG. 7B, bychoice of the wafer surface orientation and the layout of the fin withrespect to the notch it is possible to obtain a fin with all surfacebeing <100> or a fin with sidewalls being (110).

Referring now to FIGS. 8-18A and 18B, there is illustrated anotherembodiment of the present disclosure in which graphene nanoribbons areformed on sidewalls of a silicon fin. Specifically, FIGS. 8-18A and 18Bprovide a method of fabricating dual-channel finFETs, which can beoptionally double gated.

Referring first to FIG. 8, there is illustrated a silicon-on-insulatorsubstrate 50 that can be employed in this embodiment of the presentdisclosure. The silicon-on-insulator substrate 50 includes, from bottomto top, a handle substrate 52, a buried insulating layer 54 and asilicon layer 56. It is observed that the silicon-on insulator substrate50 shown in FIG. 8 is similar to the silicon carbide-on-insulatorsubstrate 10 shown in FIG. 1 except that a silicon layer 56 is used inplace of the silicon carbide layer 16. As such, the materials andthicknesses for the handle substrate 52 and the buried insulating layer54 used in this embodiment of the present disclosure are the same asthose mentioned above for handle substrate 12 and buried insulatinglayer 14 of the silicon carbide-on-insulator substrate 10. It is alsonoted that the general description of doping, crystal orientation, andthickness given above for the silicon carbide layer 16 are applicablehere for the silicon layer 56.

Also, the silicon-on-insulator (SOI) substrate 50 can be made using oneof the techniques mentioned above in forming the silicon carbide-oninsulator-substrate 10 with the except that silicon is used in place ofsilicon carbide. Furthermore the making of SOI wafers is a maturetechnology and SOI wafers are available commercially.

Referring now to FIG. 9, there is depicted the silicon-on-insulatorsubstrate 50 of FIG. 8 after forming a hard mask 58 on an upper surfaceof the silicon layer 56. The hard mask 58 that is employed in thisembodiment of the present disclosure can include one of the hard maskmaterials mentioned above for hard mask 30. Also, hard mask 58 thatemployed in this embodiment of the present disclosure may be made usingone of the techniques mentioned above for forming hard mask 30 and thethickness of hard mask 58 may fall within the range provided above forhard mask 30.

Referring now to FIG. 10, there is illustrated the structure shown inFIG. 9 after forming at least one silicon fin 56′ on an upper surface ofthe buried insulator layer 54. Although a single silicon fin 56′ isillustrated in FIG. 10, a plurality of silicon fins 56′ can be formed onthe surface of the buried insulating layer 54 similar to the pluralityof silicon carbide fins 16′ formed in the previous embodiment of thepresent disclosure. As is shown, each silicon fin 56′ includes apatterned hard mask 58′ located on an upper surface of the silicon fin56′. Also, each silicon fin 56′ has bare sidewalls.

The silicon fin 56′ can be formed by lithography and etching.Specifically, the structure shown in FIG. 10 can be formed by firstapplying a photoresist material (not shown) to the upper surface of hardmask 58. The photoresist material, which can be a positive-tonematerial, a negative-tone material or a combination of bothpositive-tone and negative-tone materials, can be formed utilizing anyconventional deposition including, for example, spin-on coating.Following the application of the photoresist material, the photoresistmaterial is subjected to a desired pattern of radiation and thereafterthe resist material is developed utilizing any conventional resistdeveloper. With the patterned resist on the surface of the hard mask 58,the unprotected portions of the hard mask 58 and the underlying portionsof the silicon layer 56 are then removed utilizing one or more etchingprocess. The one or more etching processes can include dry etching, wetetching or any combination thereof. When dry etching is employed, one ofreactive ion etching, ion beam etching, and plasma etching can be used.When wet etching is employed, a chemical etchant that is selective inremove unprotected portions of at least the hard mask 58 can be used. Inone embodiment, RIE can be used to remove the unprotected portions ofthe hard mask 58 and the underlying portions of the silicon layer 56.

In some embodiments, the patterned resist remains atop the structureduring the entire patterning process. In other embodiments of thepresent disclosure, the patterned resist is removed from the structureafter the pattern has been transferred into the hard mask 58.Notwithstanding when the patterned resist is removed, the patternedresist is removed utilizing a conventional resist removal processingsuch as ashing.

Referring now to FIG. 11, there is illustrated the structure shown inFIG. 10 after forming silicon carbide fins 60 on the bare sidewalls ofeach silicon fin 56′. Although not shown, the patterned mask 58′ can beremoved from atop each silicon fin 56′ prior to forming the siliconcarbide fin. In such an instant, a silicon carbide fin can be formedatop the silicon fin 56′.

The silicon fin 56′ and hard mask 58′ may be removed selectively withrespect to the silicon carbide fins 60. The removal of the silicon fin60 produces a structure which is similar to the structure shown in FIG.4A where the SiC fins 16′ are formed by patterning a SiC-on-insulatorlayer. There are some differences between the two structures: The firstdifference is that the number of SiC fins 60 is double that of FIG. 4A,since each silicon fin 56′ yields two SiC fins 60. The second differenceis that the SiC fins 60 do not have a hardmask cap. One advantage of themethod for producing the SiC fins 60 is that the fin thickness isdefined by epitaxy as will be explained below. Epitaxy typically allowsmore uniform control overt the fin thickness than achieved withlithography and patterning of a SiC layer. The rest of the stepsdiscussed in reference to FIGS. 5-6 can be applied to the structure tocomplete the device fabrication. The remaining of the discussion relatedto FIGS. 11-18 will be with respect to the embodiment where the siliconfin 56′ and hard mask 58′ are kept (although they are eventually removedto form a double gate structure).

The silicon carbide fins 60 that are formed on the bare sidewalls ofeach silicon fin 56′ can be formed utilizing a selective epitaxialgrowth process. Since a selective epitaxial growth process is employed,the silicon carbide fins 60 have the same crystal orientation as that ofthe sidewall of the silicon fin 56′ from which they are grown. Theselective epitaxial growth process is typically performed at atemperature from 1200° C. to 1400° C., with a growth temperature from1325° C. to 1375° C. being more typical. In one embodiment, theselective epitaxial growth process used in forming the silicon carbidefins 60 on the sidewalls of the silicon fin 56′ includes at least oneprecursor that includes both silicon and carbon. In another embodiment,the selective epitaxial growth process used in forming the siliconcarbide fins 60 on the sidewalls of the silicon fin 58′ includes a firstprecursor that includes silicon and a second precursor that includescarbon. In any of the aforementioned embodiments, the precursor(s) canbe used alone, or admixed with an inert gas.

The silicon carbide fins 60 that are formed on the bare sidewalls of thesilicon fin 56′ have a thickness extending laterally outward from thesidewall of the silicon fin 56′ from 1 nm to 10 nm, with a thicknessfrom 1 nm to 5 nm being more typical. The height of the silicon carbidefins 60 is dependent on the height of the silicon fin 56′ that waspreviously formed.

Reference is now made to FIG. 12 which illustrates the structure of FIG.11 after forming a layer of graphene on bare sidewalls of each siliconcarbide fin 60. The layer of graphene can be referred to herein asgraphene nanoribbon 62.

The graphene nanoribbons 62 of this embodiment of the present disclosureare formed utilizing the same technique that was employed in forming thegraphene nanoribbons 32 in the previous embodiment of the presentdisclosure. That is, the graphene nanoribbons 62 of this embodiment ofthe present application can be formed on the bare sidewalls of eachsilicon carbide fin 60 by first cleaning the bare sidewalls of eachsilicon carbide fin 60 by performing a first anneal in a dilutesilane-containing ambient. The first anneal temperature andsilane-containing ambient used in forming graphene nanoribbons 32 can beused here for forming graphene nanoribbons 62.

After performing the first anneal, a second anneal is performed thatgrows graphene nanoribbons 62 on the bare sidewalls of each siliconcarbide fin 60. The second anneal temperature is within the rangementioned above for forming graphene nanoribbons 32, but is kept lowerthan 1414° C. which is the melting temperature for silicon. During thesecond anneal, silicon is release from the bare sidewalls of the siliconcarbide fins 60 forming graphene nanoribbons 62 thereon. Each graphenenanoribbon 62 that is formed has a thickness extending laterally outwardfrom the surface of silicon carbide fin 60 from one monolayer to sixmonolayers, with one or two monolayers being more typical. The height ofeach graphene nanoribbon 62 is determined by the height of both thesilicon carbide fins 60.

Referring to FIGS. 13-14, there are illustrated the structure shown inFIG. 12 after forming a first gate structure 65 including a first gatedielectric 64 and a first gate conductor 66 thereon. The first gatedielectric 64 and the first conductor 66 shown in FIGS. 13 and 14include materials and thicknesses mentioned above for forming the gatedielectric and the gate conductor 34 in the previous embodimentmentioned above. Also, the first gate dielectric 64 and the first gateconductor 66 shown in FIGS. 13 and 14 are formed utilizing one of theprocesses mentioned above in forming the gate dielectric and the gateconductor 34 in the previous embodiment of the present disclosure.

The structure illustrated in FIGS. 13-14 includes at least one siliconfin 56′ located on a surface of a substrate i.e., the buried insulatinglayer 54 of the initial silicon-on-insulator substrate 50. The disclosedstructure also includes a silicon carbide fin 60 located on each baresidewall of the at least one silicon fin 56′, and a graphene nanoribbon62 located on a sidewall of each silicon carbide fin 60. The disclosedstructure further includes a gate structure 65 oriented perpendicular toeach silicon carbide fin 60 and the at least one silicon fin 56′. Thegate structure 65 also overlaps a portion of each graphene nanoribbon 62and is located atop a portion of each of the silicon carbide fins 60 andthe at least one silicon fin 56′. The portion of the each graphenenanoribbon 62 overlapped by the gate structure 65 defines a channelregion of the semiconductor structure.

Referring now to FIG. 15, there is illustrated the structure shown inFIG. 14 after forming a planarizing dielectric layer 68 and planarizingthe structure stopping on an upper surface of the patterned hard mask58′. The planarizing dielectric layer 68 employed in this embodiment ofthe present disclosure may include a photoresist material, SiO₂, a dopedsilicate glass, a silsesquioxane, a C doped oxide (i.e.,organosilicates) that include atoms of Si, C, O and H (SiCOH or porouspSiCOH), SiN, SiC:H, SiCN:H, thermosetting polyarylene ethers, ormultilayers thereof. The term “polyarylene” is used in this applicationto denote aryl moieties or inertly substituted aryl moieties which arelinked together by bonds, fused rings, or inert linking groups such as,for example, oxygen, sulfur, sulfone, sulfoxide, carbonyl and the like.

The planarizing dielectric layer 68 can be formed utilizing anyconventional deposition process including, for example, spin-on coating,chemical vapor deposition, chemical enhanced vapor deposition andchemical solution deposition. The thickness of the planarizingdielectric layer 68 that is formed prior to planarization varies so longas the upper surface of gate structure 65 that is located above thepatterned hard mask 58′ is covered with the planarizing dielectricmaterial 68.

After forming the planarizing dielectric layer 68, the planarizingdielectric layer 68 is planarized stopping atop an upper surface of thepatterned hard mask 58′. The planarizing step used in forming thestructure shown in FIG. 15 can include chemical mechanical planarizationand/or grinding. The planarization process provides a structure such asshown in FIG. 15 in which the upper surfaces of the planarizingdielectric layer 68, the first gate conductor 66, the hard mask 58′ andthe first gate dielectric 64 are each coplanar with each other.

Referring now to FIG. 16, there is shown the structure of FIG. 15 afterselectively removing the patterned hard mask 58′ and the silicon fin 56′from the structure, and formation of a second gate conductor 70 (thesecond gate conductor 70 represents a second gate line of the structure)in at least the area previously occupied by the patterned hard mask 58′and the silicon fin 56′.

The patterned hard mask 58′ and the silicon fin 56′ can be removedutilizing one or more selective etching processes. That is, thepatterned hard mask 58′ and the silicon fin 56′ can be selectivelyremoved utilizing a single etching step, or multiple etching steps canbe used to selectively remove first the patterned hard mask 58′ and thenthe silicon fin 56′. In one embodiment of the present disclosure, a wetetch can be used to selectively remove the patterned hard mask 58′ fromthe structure, stopping atop the silicon fin 56′, and thereafter RIE canbe used to selectively remove the silicon fin 56′ from the structure.More specifically an HBr based chemistry can be used to etch the siliconfin 56′ selectively with respect to the planarizing dielectric material68 and the first gate dielectric 64.

After selectively removing the patterned hard mask 58′ and the siliconfin 56′ from the structure, the second gate conductor 70 is formed in atleast the area previously occupied by the patterned mask 58′ and thesilicon fin 64′; the second gate conductor 70 can also extend onto anupper surface of the first gate conductor 66 and an upper surface of theplanarizing dielectric layer 68.

The second gate conductor 70 may comprise the same or differentconductive material as the first gate conductor 66. Also, the gateconductor 70 can be formed utilizing one of the deposition processesmentioned above for the first gate conductor 66 and after deposition thedeposited conductive material can be patterned by lithography andetching forming the second gate conductor 70 such as shown in FIG. 16.The structure shown in FIG. 16 is a double-gate FET with graphenechannels.

Reference is now made to FIG. 17, which represents another possiblestructure that can be formed utilizing the basic processing steps ofthis embodiment. Specifically, the structure shown in FIG. 15 is firstformed and then the patterned hard mask 58′ and the silicon fin 60′ areselectively removed from the structure utilizing one or more etchingprocess as described above in regard to the structure shown in FIG. 16.After selectively removing the patterned hard mask 58′ and the siliconfin 56′ from the structure, a second gate structure 71 including asecond gate dielectric 72 and second gate conductor 70 is formed in atleast the area previously occupied by the patterned hard mask 58′ andthe silicon fin 56′; a portion of second gate conductor 70 can extendonto an upper surface of the planarizing dielectric layer 68 and anupper surface of first gate conductor 66. The second gate dielectric 72abuts sidewalls of each silicon carbide fin 60 and sidewalls of thefirst gate dielectric 64. Also, in this structure, a lower portion ofthe second gate conductor 70 abuts an upper surface of the buriedinsulating layer 54.

The second gate dielectric 72 can include one of the dielectricmaterials mentioned above for the first gate dielectric 64. In oneembodiment, the second gate dielectric 72 is a different gate dielectricmaterial than the first gate dielectric 64. In yet another embodiment,the second gate dielectric 72 and the first gate dielectric 64 arecomposed of the same dielectric material. The second gate dielectric 72can be formed utilizing one of the process mentioned above that is usedin forming the first gate dielectric 64.

The second gate conductor 70 can include one of the conductive materialsmentioned above for the first gate conductor 66. In one embodiment, thesecond gate conductor 70 is a different conductive material than thefirst gate conductor 66. In yet another embodiment, the second gateconductor 70 and the first gate conductor 66 are composed of the sameconductive material. The second gate conductor 70 can be formedutilizing the process mentioned above for forming the first gateconductor 66.

Reference is now made to FIGS. 18A-18B, which represent other possiblestructures that can be formed utilizing the basic processing steps ofthis embodiment. The structures shown in FIGS. 18A-18B are double gateFETs with graphene channels. In the embodiment depicted in FIG. 18A, thefirst and second gate conductors are electrically connected. In theembodiment depicted in FIG. 18B, the first and second gate conductorsare not electrically connected. Both structure 18A and 18B can be formedby first providing the structure shown in FIG. 15. Next, the patternedhard mask 58′ and the silicon fin 56′ are selectively removed from thestructure utilizing one or more etching processes as described above inregard to the structure shown in FIG. 16.

After selectively removing the patterned hard mask 58′ and the siliconfin 56′ from the structure, the silicon carbide fins 60 are selectiveremoved from the structure utilizing an isotropic etching process suchas, for example, hot phosphoric (H₃PO₄ at 180 C), or plasma etch withSF₆.

After selectively removing the silicon carbide fins 60 from thestructure, a second gate structure 71 including a second gate dielectric72 and a second gate conductor 70 is formed in at least the areapreviously occupied by the silicon carbide fins 60, the patterned hardmask 58′ and the silicon fin 56′. In one embodiment and as shown in FIG.18A, a portion of second gate conductor 70 can extend onto an uppersurface of the planarizing dielectric layer 68 and an upper surface offirst gate conductor 66. In the embodiment shown in FIG. 18A, the twogates are electrically connected. In another embodiment and as shown inFIG. 18B, the second gate conductor 70 does not extend onto the uppersurface of at least the first gate conductor 66. In the embodiment shownin FIG. 18B, the two gates are electrically separated. In eitherstructure, the second gate dielectric 72 abuts sidewalls of the graphenenanoribbons 62 and sidewalls of the first gate dielectric 64. Also, inthese structures, the second gate dielectric 72 lies beneath the secondgate conductor 70. As such, a lower portion of the second gate conductor70 is separated from buried insulating layer 54 by a portion of thesecond gate dielectric 72.

The second gate dielectric 72 can include one of the dielectricmaterials mentioned above for the first gate dielectric 64. In oneembodiment, the second gate dielectric 72 is a different gate dielectricmaterial than the first gate dielectric 64. In yet another embodiment,the second gate dielectric 72 and the first gate dielectric 64 arecomposed of the same dielectric material. The second gate dielectric 72can be formed utilizing one of the process mentioned above that is usedin forming the first gate dielectric 64.

The second gate conductor 70 can include one of the conductive materialsmentioned above for the first gate conductor 66. In one embodiment, thesecond gate conductor 70 is a different conductive material than thefirst gate conductor 66. In yet another embodiment, the second gateconductor and the first gate conductor 66 are composed of the sameconductive material. The second gate conductor 70 can be formedutilizing the process mentioned above for forming the first gateconductor 66.

The structures shown in FIGS. 16-18A and 18B include at least one pairof spaced apart graphene nanoribbons located on a surface of asubstrate, i.e., the buried insulating layer 54 of the originalsilicon-on-insulator substrate. This structure also includes a firstgate structure 65 located on one sidewall of each spaced apart graphenenanoribbon, wherein the sidewalls of each graphene nanoribbon containingthe first gate structure 65 are not facing each other. The structurefurther includes a planarizing dielectric material 68 located adjacentthe first gate structure 65, and at least a gate conductor 70 of asecond gate structure 71 located between the at least one pair of spacedapart graphene nanoribbons. In some embodiments, an upper portion of thesecond gate conductor 70 of the second gate structure 71 can contact anupper surface of the first gate structure 65, while in others the secondgate conductor 70 does not contact an upper surface of the firststructure 66.

Referring now to FIGS. 19A, 19B, 20A, 20B, 21A and 21B, there isillustrated another embodiment of the present disclosure in which carbonnanotubes are formed from silicon carbide nanowires. Specifically, thisembodiment of the present disclosures provides a method of forming agate-all-round carbon nanotube FET.

This embodiment begins by first providing the silicon carbideon-insulator substrate 10 shown in FIG. 1. Next, a plurality ofsuspended silicon carbide nanowires 80 is formed in at least one regionof the structure providing a structure such as shown, for example, inFIGS. 19A-19B. Although a plurality of suspended silicon carbidenanowires oriented in a ladder type array arrangement is described andillustrated, the present application also contemplates an embodimentwhen a single suspended carbon nanowire is formed.

The suspended silicon carbide nanowires 80 are formed by lithography,etching and recessing portions of the buried insulating layer 14 frombeneath each silicon carbide nanowire that is formed. Each suspendedsilicon carbide nanowire 80 has upper, lower and sidewalls surfaces thatare bare. As is illustrated, the plurality of suspended silicon carbidenanowire 80 have a first end portion E1 that is in contact with a firstunpatterned portion of the silicon carbide layer 16, and a second endportion E2 that is in contact with a second unpatterned portion of thesilicon carbide layer 16. As also illustrated, the plurality ofsuspended silicon carbide nanowires 80 are arranged parallel to eachother and a uniform space is present between each neighboring siliconcarbide nanowire 80.

As mentioned above, the structure shown in FIGS. 19A-19B can be formedby lithography and etching a plurality of unsuspended silicon carbidenanowires and thereafter removing portions of the buried insulatinglayer from beneath each unsuspended nanowire. Specifically, thestructure shown in FIGS. 19A-19B is formed by first applying aphotoresist material (not shown) to the upper surface of silicon carbidelayer 16. The photoresist material, which can be a positive-tonematerial, a negative-tone material or a combination of bothpositive-tone and negative-tone materials, can be formed utilizing anyconventional deposition including, for example, spin-on coating.Following the application of the photoresist material, the photoresistmaterial is subjected to a desired pattern of radiation and thereafterthe resist material is developed utilizing any conventional resistdeveloper. With the patterned resist on the surface of the siliconcarbide layer 16, the unprotected portions of silicon carbide layer 16are then removed utilizing an etching process. The etching process caninclude dry etching or wet etching. When dry etching is employed, one ofreactive ion etching, ion beam etching, and plasma etching can be used.When wet etching is employed, a chemical etchant that is selective inremove unprotected portions of the silicon carbide layer 16 can be used.In one embodiment, SF₆ based chemistry can be used to etch unprotectedportions of the silicon carbide layer 16 not covered with the patternedresist. After patterning the silicon carbide layer 16, the patternedresist is removed utilizing a conventional resist removal processingsuch as ashing.

After forming the array of unsuspended silicon carbide nanowires, theburied insulating layer 14 beneath each carbon nanowire is removedutilizing an isotropic etching process such as, for example, a wet etch.More specifically, if the buried insulating layer 14 is SiO₂, thendiluted HF (DHF) can be used to selectively undercut and suspend thenanowires.

Each suspended silicon carbide nanowire 80 that is formed has a lengthfrom 5 nm to 200 nm, with a length from 20 nm to 100 nm being moretypical. The height of each suspended silicon carbide nanowire 80 isdepended on the thickness of the original silicon carbide layer 16. Theterm “nanowire” as used throughout this application denotes arectangular bar with a width and height dimensions that are severaltimes smaller than the length dimension. Since the wire dimensions aretypically in the nanometer scale it is referred to as a nanowire.

Referring now to FIGS. 20A-20B, there is shown the structure of FIGS.19A-19B after forming a graphene coating 82 on all exposed surfaces ofthe plurality of suspended silicon carbide nanowires 80; the nanowirescoated with graphene may be referred to herein as carbon nanotubes 84.It is also observed that a graphene coating 82′ forms on the uppersurface of the silicon carbide layer 16 which was previously notpatterned into suspended silicon carbide nanowires 80. The areasincluding the silicon carbide layer 16 that is coated with graphenecoating 82′ can be processed into the source and drain regions of thestructure. The area in which the source and drain regions aresubsequently formed are labeled as element 88 in the subsequentdrawings.

The graphene coating 82, 82′ is formed utilizing the same techniquementioned above for forming graphene nanoribbon 32. That is, the exposedsilicon carbide nanowire surfaces are first cleaned by annealing in adilute silane ambient. After cleaning the exposed surfaces of thesilicon carbide nanowires, a second anneal is used to form a graphenecoating on all exposed silicon carbide surfaces.

Referring now to FIGS. 21A-21B, there is illustrated the structure shownin FIGS. 20A-20B after forming a gate structure 89 including a gatedielectric (not shown) and a gate conductor 90 over a portion of eachcarbon nanotube 84. The gate dielectric employed in this embodiment caninclude one of the gate dielectric materials mentioned above in theregard to FIGS. 6A-6B. Also, gate conductor 90 can include one of theconductive material mentioned above for gate conductor 34. The gatedielectric and the gate conductor 90 of this embodiment can be formedutilizing one of the processes mentioned above for forming the gatedielectric and gate conductor 34 in FIGS. 6A-6B. The gate dielectric andthe gate conductor are surrounding the suspended carbon nanotube andform a gate-all-around structure.

The structure shown in FIGS. 21A-22B include at least one suspendedcarbon nanotube 84 located atop a surface of a substrate, i.e., theburied insulator layer 14 of the initial silicon carbide-on-insulatorsubstrate 10. The structure further includes a gate structure 89oriented perpendicular to the at least one suspended carbon nanotube 84.The gate structure 89 also surrounds a portion of the at least onesuspended carbon nanotube 84, and portions of the at least one carbonnanotube 84 surrounded by the gate structure 88 define a channel regionof the semiconductor structure.

Source and drain regions 88 can be formed in the graphene regions overthe non-patterned portions of the silicon carbide layer and the portionof the carbon nanotube that extends outside the gate region. The carbonnanotube outside the gate region and the graphene over the unpatternedSiC can be doped by chemical doping and can be reacted to form a metalcarbide such as WC.

While the present disclosure has been particularly shown and describedwith respect to various embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A semiconductor structure comprising: at least one silicon carbide fin having a bottommost surface directly contacting a portion of an uppermost surface of an insulator layer; a graphene nanoribbon located on each bare sidewall of said at least one silicon carbide fin; and a gate structure oriented perpendicular to said at least one silicon carbide fin, said gate structure overlapping a portion of each graphene nanoribbon and located atop a portion of said at least one silicon carbide fin, wherein the portion of the each graphene nanoribbon overlapped by said gate structure defines a channel region of the semiconductor structure.
 2. The semiconductor structure of claim 1 wherein said at least one silicon carbide fin comprises a plurality of parallel oriented silicon carbide fins, each silicon carbide fin of said plurality of parallel oriented silicon carbide fins having said graphene nanoribbon located on bare sidewalls thereof.
 3. The semiconductor structure of claim 1 wherein each graphene nanoribbon has a width defined by a height of said at least one silicon carbide fin.
 4. The semiconductor structure of claim 1 wherein one portion of each graphene nanoribbon not overlapped by said gate structure is a source region of the semiconductor structure, and wherein another portion of each graphene nanoribbon not overlapped by said gate structure is a drain region, and wherein said source region and said drain region are connected by the channel region.
 5. The semiconductor structure of claim 4 wherein said source region and said drain region each include a metal carbide.
 6. The semiconductor structure of claim 4 wherein said source region and said drain region are doped.
 7. The semiconductor structure of claim 1 wherein one end of said at least one silicon carbide fin is in contact with a first silicon carbide layer portion and another end of said at least one silicon carbide fin is in contact with a second silicon carbide layer portion.
 8. The semiconductor structure of claim 1 wherein said graphene nanoribbon is in direct physical contact with each bare sidewall of said at least one silicon carbide fin.
 9. The semiconductor structure of claim 1 further comprising a hard mask located on an uppermost surface of said at least one silicon carbide fin.
 10. The semiconductor structure of claim 1 wherein said graphene nanoribbon has a bottommost surface in direct contact with another portion of said insulator layer.
 11. The semiconductor structure of claim 1 wherein said graphene nanoribbon has a 2D hexagonal crystallographic structure.
 12. The semiconductor structure of claim 1 wherein said graphene nanoribbon comprises graphene having an amorphous carbon phase.
 13. The semiconductor structure of claim 1 wherein said graphene nanoribbon comprises graphene having a disordered carbon phase.
 14. The semiconductor structure of claim 1 wherein said graphene nanoribbon comprises graphene having an amorphous carbon phase and a disordered carbon phase.
 15. The semiconductor structure of claim 1 wherein said graphene nanoribbon comprises graphene having at least one of substitutional dopant species, interstitial dopant species and intercalated dopant species.
 16. The semiconductor structure of claim 1 further comprising a handle substrate located beneath said insulator layer.
 17. The semiconductor structure of claim 1 wherein said insulator layer is an oxide, a nitride, or an oxynitride. 